A way of denoting a chemical reaction using the symbols for the participating particles (atoms, molecules, ions, etc.); for example,xA+yB → zC+wD The single arrow is used for an irreversible reaction; double arrows (⇌) are used for reversible reactions. When reactions involve different phases it is usual to put the phase in brackets after the symbol (s = solid; l = liquid; g = gas; aq = aqueous). The numbers x, y, z, and w, showing the relative numbers of molecules reacting, are called the stoichiometric coefficients. The sum of the coefficients of the reactants minus the sum of the coefficients of the products (x+y – z – w in the example) is the stoichiometric sum. If this is zero the equation is balanced. Sometimes a generalized chemical equation is considered ν1A1+ν2A2+… → … νnAn+νn+1An+1 … In this case the reaction can be written ΣνiAi = 0, where the convention is that stoichiometric coefficients are positive for reactants and negative for products. The stoichiometric sum is Σνi.
xA+yB → zC+wD
ν1A1+ν2A2+… → … νnAn+νn+1An+1 …